Compositions and methods for high temperature stable catalysts

ABSTRACT

Catalysts having good high temperature stability which are particularly useful for selected high temperature reactions are disclosed as are methods for their preparation and use. The catalytically-active materials include platinum group metal deposited on a catalytic slip or composite which contains a mixture of alumina, a rare earth metal oxide, and a metallic oxide wherein the metal is IVB, selected VIB metals, and mixtures thereof. The slips or carrier compositions are calcined at a temperature of at least 850°C. before deposition of platinum group metal and characterized by having a surface area of at least 20 m 2  /g after calcination at a temperature of 1200°C. for two hours.

The present invention relates to catalyst compositions and methods fortheir preparation and use. In particular, this invention relates tocatalyst compositions characterized by high stability therebymaintaining good catalytic activity.

Catalyst compositions exhibit a relatively high surface area per unitweight to allow the largest amount of reactants to contact the catalyst.Additionally, high surface area is important when the catalystcomposition contains a precious metal such as platinum because of thecost of the metal and because of the dispersion required to preventundue metal crystallite growth. It is desirable to retain this highsurface area for long periods of use under severe conditions which mightinclude reaction temperatures of 1200°C. or higher.

Alumina is an excellent and relatively economical carrier or support formany catalysts. Many crystalline forms of alumina, for example, chi,kappa, gamma, delta, eta, and theta, exhibit a very high surface area inrelation to their weight. A serious drawback of alumina as a catalystcarrier, however, is its transition temperature of about 1000°-1200°C.to the alpha form which results in a substantial reduction of thesurface area. It is thus extremely desirable to stabilizealumina-containing catalyst compositions based on high surface areaaluminas to substantially prevent the transition to the low surfacealpha form with a consequent loss in activity.

It is therefore an object of this invention to provide catalystcompositions, as well as methods for their preparation and use, whichexhibit high temperature stability. Other objects and advantages willappear as the description proceeds.

Broadly, the catalyst composition of this invention includes acatalytically-active, calcined composite characterized by a surface areaof at least 20 square meters per gram (m² /g) after calcination for twohours at a temperature of 1200°C., said composite comprising or being acomposite of alumina, a rare earth metal oxide and a metal oxide whereinthe metal is selected from the group consisting of chromium, tungsten, aGroup IVB metal and mixtures thereof. In preparing the catalystcomposition, the composite is first calcined at a temperature of atleast 850°C. and then a catalytically-effective amount of a platinumgroup metal is added to the composite. A catalyst composition preparedin accordance with this invention exhibits high temperature stabilityand therefore catalytic activity in a number of high temperaturereactions, particularly high temperature combustion reactions.

The composite is formed by the calcination of an intimate admixture ofan aluminum compound, rare earth metal compound and at least one metalcompound wherein the metal is selected from the group consisting ofchromium, tungsten, a Group IVB metal and mixtures thereof. Preferably,for certain methods of preparation, the aluminum compound is alumina.These compounds, as indicated, if not already in oxide form must becapable of forming or yielding their respective oxides upon calcinationin air (oxygen) at a temperature of at least 850°C. The combination ofthe rare earth metal oxide and the other metal oxide or oxides may beconsidered as a high temperature stabilizing component for the alumina.

The relative amounts of alumina to the metal oxide stabilizingcomponent, that is, the rare earth metal oxide and oxides of the metalsof the Group IVB metals and chromium and tungsten and/or mixtures ofthese compounds, are governed largely by empirical criteria. While it isnot desired that this invention be limited by the following theory, abrief statement may provide a helpful framework to further elucidate theinvention. It is thought that the addition of the stabilizing componentto the alumina or alumina precursor and calcination of the mixture at atemperature of at least 850°C. converts any of the non-oxide compoundsto oxides and allows the stabilizing component oxides to enter thealumina lattice and prevent or substantially reduce subsequenttransition to alpha alumina.

All surface areas throughout the specification and the appended claimsare measured by the B.E.T. or equivalent method. The terminology used todescribe the metals herein, that is, the rare earth or lanthanide seriesand the Group IVB metals, is the terminology used in association withthe common long form of the Periodic Table of Elements. Thus the GroupIVB metals are titanium, zirconium, and thorium hafnium and the rareearth or lanthanide metals are metals of atomic number 57 to 71.

The catalyst composition may also contain a minor amount of otheringredients, up to about 5 percent by weight of the composite, which mayserve as promoters, activators, or other purposes, for oxidation orreduction reactions. Such ingredients may include, for example,manganese, vanadium, copper, iron, cobalt, and nickel usually as themetal oxide or sulfide.

The calcined composite may be formed to any desired shape such as apowder, beads, or pellets. This shaping or fabricating is accomplishedbefore calcination to promote particle adhesion. After calcination, aplatinum group metal is added to the composite. Additionally, thecomposite can be applied or deposited on a relatively inert support orsubstrate and the platinum group metal then added, or the catalystcomposition ca be applied or deposited onto the inert support.

For compositions made in accordance with this invention, the compositegenerally comprises about 50 to 95 weight percent alumina, and about 2to 25 weight percent of rare earth metal oxide, preferably about 5 to 15weight percent, based on the total weight of composite. The Group IVBmetal oxide, if used alone with the rare earth metal oxide, may bepresent in about 2 to 25 weight percent of the composite, preferablyabout 5 to 15 weight percent, but if used in combination with chromiumand/or tungsten oxide may be present in about 2 to 15 weight percent,preferably about 5 to 15 weight percent of the composite. The Cr or Woxide may be present in about 2 to 25 weight percent, preferably about 5to 15 weight percent of the composite. Mixtures of Group IVB metaloxides and chromium and/or tungsten oxides may be present in about 5 to30 weight percent, preferably about 5 to 15 weight percent of thecomposite. If the amount of alumina is too low, the resulting compositewill not provide enough surface area to provide catalytic activity. Ifmore alumina is present than stated, it may not be stabilizedsufficiently and will lose surface area in the transition to the alphaform.

Generally, to provide the advantages of this invention, it is necessaryfor the stabilizing component to be in intimate association with thealumina during pre-calcining. An intimate admixture may be achieved, forexample, by forming a slurry of alumina with water soluble compounds ofthe stabilizing components. Where desired, hydrated alumina, such asaluminum trihydrate is admixed with aqueous solutions of a rare earthmetal salt and at least one of the other metal salts of this inventionto permit sorption of the stabilizing components by the alumina. Thesolids are then recovered from the slurry and calcined to provide themixed oxide composite. The particulate alumina is preferably in finelydivided or colloidal form to provide maximum sorption area. For example,finely divided freshly precipitated aluminum trihydrate having aparticle size of 70 percent to 90 percent smaller than 325 mesh isuseful. When large particle size alumina is used, the sorption of thestabilizing components from solution and subsequent calcination willprovide at least a stabilized outer portion of the alumina.

Another method of preparing intimate admixture of alumina andstabilizing components is to coprecipitate all of the components,including the alumina, from aqueous solutions. Various methods ofcoprecipitation are suitable. Such methods include, for example, surfaceadsorption where one or more components in ionic form are sorbed on thesurface of a precipitating solid; and inclusion, in which thecoprecipitated compound or compounds have dimensions and a chemicalcomposition which will fit into the crystal structure of a precipitatingsolid without causing appreciable distortion.

In coprecipitation, a suitable precipitant, usually a base, is added toan aqueous solution of the compounds. This can also be done byconcurrent addition of both the precipitant and the compound solution toa vessel containing water. Preferably the precipitant is selected suchthat undesirable or unnecessary compounds are volatilizable anddecomposable upon calcination at 850°C. or above, or removable bywashing or extraction. The precipitant is capable of initiating andcompleting essentially simultaneous coprecipitation of the components.Suitable precipitants are ammonium compounds such as ammonium hydroxideor ammonium carbonate as well as other hydroxides and carbonates of thealkali metals.

The precipitant may be in dilute or concentrated aqueous solution. Therapidity of addition of the precipitant and the degree of agitation usedwill vary depending upon the precipitate desired. Dilute precipitantsolutions, slow addition, and vigorous agitation generally favor acoarser precipitate. The temperature during the addition of precipitantmay be from about 0° to 90°C. Higher temperatures generally produce acoarser precipitate. The precipitant is added until a pH of about 5 to9.0 is reached. At this time the coprecipitated mixture is recoveredfrom the slurry, washed if desired, and digested or recrystallized ifdesired.

The intimate admixture of alumina and stabilizing components arecalcined at a temperature of at least about 850°C., preferably about900°to 1200°C., but not at such a high temperature or for such a longperiod of time to unduly sinter the composite. The conditions of thecalcination are such as to provide a catalytically-active compositehaving a relatively high surface area of at least about 25 square metersper gram, and preferably at least about 75. Calcination is preferablyconducted while the admixture is unsupported and in free-flowingcondition. This is preferable for economic reasons and to prevent unduesintering.

Calcination in air to form the composite, and prior to the addition of aplatinum group metal, is an integral part of the subject invention. Itis found that an intimate admixture of the stabilizing components andthe alumina is stable when calcined at such temperatures before anyfurther preparative steps are performed. Since both the alumina and thestabilizing components are intimately admixed, the concurrent heating inclose association substantially reduces any undesirable aluminatransitions. Additionally, calcination before deposit on an inertsubstrate promotes adhesion of the calcined composite to the substratethus allowing the use of higher space velocities with the finishedcatalyst composition with less chance of erosion. Further, calcinationsubstantially reduces the possibility of reaction of the stabilizingcomponent and alumina component with the substrate. Any such reactionsbetween the alumina and the substrate promotes the formation of inactiveforms of alumina thereby reducing its surface area and activity. If thestabilizing component were to react with the substrate, it would reducethe effective amount of this component available for stabilization. Afurther advantage of such calcination is economic because less heat insmaller furnaces is required to calcine the resulting powder compositebefore it is placed on an inert support. Further, it is essential thatthe calcination is conducted before the addition of a platinum groupmetal component to prevent loss of such component by occlusion.

Suitable aluminum-containing compounds are alumina, the gamma, eta,kappa, delta, and theta forms of alumina and for coprecipitation, thewater soluble aluminum compounds such as salts, for example, thealuminum halides, aluminum nitrate, aluminum acetate, and aluminumsulfate.

The rare earth metal compounds which may be employed to produce thecatalytic composite are, for example, the compounds of cerium,lanthanum, neodymium, samarium, praseodymium, and the like as well ascommercially available mixtures of rare earths. The rare earth used ispreferably cerium. If a mixture of rare earths is used, the mixture ispreferably one in which cerium is the predominant component. Suitablewater soluble rare earth metal compounds include the acetates, halides,nitrates, sulfates, and the like, e.g., Ce(C₂ H₃ O₂)₃, CeBr₃, Ce(NO₃)₃,Ce₂ (SO₄)₃, Nd(C₂ H₃ O₂)₃, Sm(NO₃)₃, and TmBr₃.

The Group IVB metal oxides, i.e., the oxides of titanium, thoriumzirconium, and hafnium, are added to the alumina in the form of theirwater soluble precursors. Thus, for example, water soluble IVB metalsalts such as the nitrates, acetates, halides, and sulfates and the likemight be employed. Suitable water soluble compounds are Zr(NO₃)₄, ZrCl₄,Zr(SO₄)₂, ZrOCl₂, Ti₂ (C₂ O₄)₃, and HfOCl₂.

Water soluble compounds of chromium and tungsten which can be used are,for example, chromium acetate, chromium nitrate, chromium halides,chromium oxide (chromic acid), chromium oxalate, and complexes ofchromium such as chloropentamine chromium chloride, tungsten halides,tungsten oxy-salts, such as tungsten dioxydichloride, ammomiumtungstate, and the like.

A platinum group metal is added to the calcined composite to form thecatalyst compositions of this invention, which are found to be effectivefor long time high temperature reactions. Such metals are usually addedor incorporated in amounts sufficient to provide significant activity.The platinum group metals useful are platinum, ruthenium, palladiumiridium, and rhodium. The choice of metal, metal combinations or alloysis governed largely by activity, specificity, volatility, deactivationby specific components included with the reactants, and economics.

The quantity of platinum group metal added to the calcined compositedepends first on design requirements such as activity and life andsecond on economics. Theoretically, the maximum amount of such metal isenough to cover the maximum amount of surface available without causingundue metal crystallite growth and loss of activity during use. Twomajor competing phenomena are involved in such surface treatment. It isdesirable to completely cover the substrate surface to provide thegreatest amount of platinum group metal coverage, thereby obtainingmaximum activity, but if the surface were to be completely covered, suchcoverage would promote growth between adjacent crystallites, whichgrowth would then decrease the surface area and greatly reduce activity.A balance of maximum coverage coupled with proper dispersion thus mustbe achieved to formulate a practical catalyst. An ancillaryconsideration in relation to the amount of platinum group metal is theallowable size of the catalyst housing. If the size is small, the amountof platinum group metal component used is preferably increased withinthe above-described limits. For example, for automobile exhausttreatment, the allowable size is relatively small, especially if unitaryhoneycomb type supports are used and a higher loading may be desirable.Economics, of course, dictates the use of the least amount of platinumgroup metal component possible while accomplishing the main objective ofpromoting the reaction. Generally, the amount of platinum group metalused is a minor portion of the catalyst composite and typically does notexceed about 20 weight percent of the calcined composite. The amount maybe about 0.1 to 20 percent and is preferably about 0.2 to 10 percent toeconomically maintain good activity with prolonged use. Thesepercentages are based on the weight of the calcined composite. If thecomposite is used on an inert substrate, the composite may be, forexample, about 10 percent of the weight of the substrate and the percentweight of platinum group metal in relation to the total weight ofsubstrate and composite will be correspondingly less.

During preparation of the catalyst composition, various compounds and/orcomplexes as well as elemental dispersions of any of the platinum groupmetals may be used to achieve deposition of the metal on the composite.Water soluble platinum group metal compounds or complexes may be used.The platinum group metal may be precipitated from solution, for example,as a sulfide by contact with hydrogen sulfide. The only limitation onthe carrier liquids is that the liquids should not react with theplatinum group metal compound and be removable by volatilization ordecomposition upon subsequent heating and/or vacuum, which may beaccomplished as part of the preparation or in the use of the completedcatalyst composition. Suitable platinum group metal compounds are, forexample, chloroplatinic acid, potassium platinum chloride, ammoniumplatinum thiocyanate, platinum tetrammine hydroxide, platinum groupmetal chlorides, oxides, sulfides, and nitrates, platinum tetramminechloride, palladium tetrammine chloride, sodium palladium chloride,hexammine rhodium chloride, and hexammine iridium chloride. If a mixtureof platinum and palladium is desired, the platinum and palladium may bein water soluble form, for example, as amine hydroxides or they may bepresent as chloroplatinic acid and palladium nitrate when used inpreparing the catalyst of the present invention. The platinum groupmetal may be present in the catalyst composition in elemental orcombined forms, e.g., as an oxide or sulfide. During subsequenttreatment such as by calcining or upon use, essentially all of theplatinum group metal is converted to the elemental form.

While these catalyst compositions are useful in many reactions, they arenot necessarily equivalent in all processes nor are those which areuseful in the same process necessarily exactly equivalent to each other.

While it is not essential, the catalyst compositions of this inventionpreferably have a relatively catalytically-inert support or substrate.The supports which can be employed in this invention are preferablyunitary, skeletal structures of relatively large size, e.g., honeycombs.However, smaller particle forms may be used, e.g., pellets or spheres.The size of these pellets can be altered depending upon the system, itsdesign and operating parameters in which they are to be used, but mayrange from about 1/64 to 1/2 inch, preferably 1/32 to 1/4inch, indiameter; and their lengths are about 1/64 to 1 inch, preferably about1/32 to 1/4 inch.

When a support is used, the calcined composite is generally present in aminor amount of the total catalyst composition, which is usually about 2to 30 weight percent preferably about 5 to 20 weight percent, based onthe total weight of the composite and support. The amount used dependson economics, size limitations, and design characteristics.

These supports whether of the unitary-skeletal type or pellets arepreferably constructed of a substantially inert, rigid material capableof maintaining its shape and strength at high temperatures, for example,up to about 1800°C. The support typically has a low thermal coefficientof expansion, good thermal shock resistance, and low thermalconductivity. While a support having a porous surface is preferred, thesurface may be relatively non-porous, but in such event it is desirableto roughen the surface to improve adhesion of deposited compositions.

The support may be metallic or ceramic in nature or a combinationthereof. The preferred supports, whether in skeletal or other form, arecomposed primarily of refractory metal oxide including combined oxideforms, e.g., alumino-silicates. Suitable support materials includecordierite, cordierite-alpha alumina, silicon nitride, silicon carbide,zircon-mullite, spodumene, alumina-silica-magnesia, and zirconiumsilicate. Examples of other suitable refractory ceramic materials aresillimanite, magnesium silicates, zircon, petalite, alpha-alumina, andaluminosilicates. Although the support may be a glass ceramic, it ispreferably unglazed and may be essentially entirely crystalline in formand marked by the absence of any significant amount of glassy oramorphous matrices. Further, the structure may have considerablyaccessible porosity, preferably having a water pore volume of at leastabout 10 percent. Such supports are described in U.S. Pat. No.3,565,830, herein incorporated by reference.

The geometric, superficial, or apparent surface area of the skeletal orhoneycomb type supports, including the walls of the gas flow channels isgenerally about 0.5 to 6, and preferably 1 to 5, square meters per literof support. This surface area is sufficient for deposition of asatisfactory quantity of the composite or the finished catalystcomposition. The plurality of channels, about 100 to 2500, preferably150 to 500 per square inch of cross-sectional area, may be distributedacross the entire face of the structure and frequently they define anopen area in excess of 60 percent of the total area of the support. Thewalls must be thick enough to provide rigidity and integrity to thestructure while maintaining good apparent surface area. The wallthickness is thus in the range of about 2 to 25 mils. The flow channelscan be of any shape and size consistent with the desired superficialsurface area and should be large enough to permit relatively freepassage of the gaseous reaction mixture; preferably the length of thechannels is at least about 0.1 inch to insure sufficient contact orresidence time to cause the desired reaction. Although the channels aregenerally parallel, they may be multi-directional and may communicatewith one or more adjacent channels.

In one manner of preparing structures provided with catalystcompositions of this invention, an aqueous slurry of the essentiallywater insoluble calcined composite of alumina and stabilizing componentis contacted with the support. The solid content of the slurry forms anadherent deposit on the support, and the resulting supported compositeis dried or calcined for a second time at a temperature which provides arelatively catalytically-active product. The second drying orcalcination takes place at a temperature low enough to prevent unduesintering of the mixture. Suitable calcination temperatures aregenerally about 300°-700°C. to insure catalytic activity without unduesintering, preferably about 400°-600°C. After this second calcinationthe coating on the support has a surface area of at least about 75s.m.p.g. Lower temperatures can be employed to dry the composite if thesecond calcination is not performed.

After the coated support is dried or calcined, a platinum group metalcomponent is added to enhance the catalytic activity of the composite.The platinum group metal may be added to the coated support in themanner previously described. Preferably, this addition is made from anaqueous or other solution to impregnate or deposit the platinum groupmetal component on the coated support.

After addition of the platinum group metal, the resulting structure isdried and may be calcined for a third time under conditions whichprovide a composition having characteristics that enhance selectedreactions. This final calcination stabilizes the completed catalystcomposition so that during the initial stages of use, the activity ofthe catalyst is not materially altered. The temperature of this finalcalcination must be low enough to prevent substantial sintering of theunderlying coating which would cause substantial occlusion of theplatinum group metal component. Thus the calcination may be conducted attemperatures of about 300°-700°C., preferably about 400°-600°C.

An alternative method of making the catalyst compositions of thisinvention if a relatively inert support is used involves adding theplatinum group metal component to the calcined composite before thecomposite is deposited on the support. For example, an aqueous slurry ofthe calcined composite can be prepared and the platinum group metalcomponent added to the slurry and mixed intimately therewith. Theplatinum group metal component can be in the form already described andmay be precipitated as previously described. The final mixturecontaining the platinum group metal may then be dried or calcined toprovide a catalytically-active composition in a form suitable fordeposition on a support or for use without such deposition as a finishedcatalyst in either finely divided or macrosize forms. Subsequentcalcinations or drying may be conducted as described above. The calcinedmaterial generally has a surface area of at least about 25 s.m.p.g.,preferably at least about 75 s.m.p.g.

The following are examples of the general method of preparation of somerepresentative stabilized catalytic composites and compositions of thisinvention. All percentages, parts, and proportions herein and in theappended claims are by weight unless otherwise indicated.

EXAMPLE I

A stabilized CeO₂, ZrO₂, and Al₂ O₃ composite slip is prepared bydissolving 17.82 grams of cerium nitrate and 14.41 grams of zirconylnitrate in 628 ml H₂ O to form a total volume of 632.5 ml. 275 grams ofactivated Al₂ O₃ powder is stirred into the solution with constantagitation for 10 minutes. The total solution is then evaporated todryness under heat and with agitation, transferred to a drying oven at120°C., and dried overnight. The dried solids are ground to less than 20mesh and calcined at 970°C. for one hour. Five grams of the compositehaving a density of 0.476 g/cc and containing 3.3% ceria, 3.3% zirconia,and 93.4% alumina, is then tested for retention of surface area bycalcining at 1200°C. for 2 hours. It is found that the surface areaafter such calcination is 36.6 m² /g.

EXAMPLE II

186 grams of the calcined powder from EXAMPLE I are mixed with 286 ml.H₂ O and 13.9 ml. conc. HNO₃, and ball-milled for 19 hours at 68 RPM ina U.S. Stoneware 1-gallon mill jar. 330 ml. of the resulting slip havinga density of 1.4 g/cc and a pH of 4.45 are diluted with 30 ml of waterto a viscosity of about 68 cps. A 20 cubic inch cordierite honeycombhaving about 250 parallel gas passages per square inch ofcross-sectional area is dipped into this diluted slip, drained, blownwith air, dried at 120°C. for 2 1/2 hours, and calcined at 500°C. for 2hours. The adherent composite makes up approximately 17 weight percentof the coated honeycomb.

EXAMPLE III

A honeycomb, coated with a ceria-zirconia-alumina composite slip isprepared as in EXAMPLE II. The coated honeycomb is then dipped intoabout 420 ml. of a solution containing both H₂ PtCl₆ and Na₂ PdCl₄,concentrations of each being such that there is theoretically 0.9% Pt byweight of solution and 0.3% Pd by weight of solution. After standing for10 minutes with intermittent raising and lowering of the honeycomb intothe solution, the honeycomb is withdrawn from the solution, drained, andexcess solution blown off. The honeycomb is then treated with gaseoushydrogen sulfide for 15 minutes, and washed chloride-free usingdeionized water. The resulting impregnated honeycomb is dried overnightat 110°C., and calcined in flowing air for 2 hours at 500°C. Thefinished catalyst contains about 0.4 weight % Pt and 0.1 weight percentPd.

EXAMPLE IV

A zircon-mullite honeycomb is coated with a composite slip containingCr₂ O₃, CeO₂ and Al₂ O₃ and then impregnated with Pt using the amminehydroxide as the platinum source. 1200 g. of activated alumina powder,less than 40 mesh in size, is slurried in a mixer with a solutionprepared by dissolving 1263 g. Cr(NO₃)₃.9H₂ O and 691 g. Ce(NO₃)₃.6H₂ Oin 156 ml. H₂ O at 75°C. A further 240 ml. of H₂ O is slowly added andthe whole mixed for 1/2 hour. At the end of this time, the mass isuniform in appearance and dark green. The mass is then dried at 110°C.,resulting large lumps are crushed, and the material is then dried for 16hours at this temperature. After drying, the solids are crushed andscreened to less than 40 mesh, and the powder is calcined for 4 hours at1000°C. 350 g. of the powder is charged to a 1/2 gallon ball mill, and350 ml. H₂ O, 7 ml. conc. HNO₃ and ceramic balls are added. The mill isrolled for 16 hours at 99 RPM. The pH of the slurry is 3.7. 300 ml. ofthe slurry are diluted with 100 ml. H₂ O containing 1 ml. conc. HNO₃. Azircon-mullite honeycomb, from American Lava Corporation, with about 100flow paths per square inch of cross-section, is dipped in the dilutedslip and held for 1 minute, then withdrawn and blown with air to removeexcess slip. The honeycomb is dried 16 hours at 110°C., and thencalcined for 2 hours at 1000°C. The cooled honeycomb shows a pickup of16.7 weight percent composite slip which has a composition ofapproximately 70 percent by weight alumina, 14 percent by weightchromia, and 16 percent by weight ceria. The coated honeycomb is thendipped in an aqueous solution of platinum tetrammine hydroxide, having0.435 g. platinum in 184 ml. of solution, for 1 minute, then the excessblown off and the catalyst dried at 110°C. After drying, the honeycombis calcined for 2 hours at 400°C. The final honeycomb contains nominally0.5 weight percent Pt.

EXAMPLE V

A zircon-mullite honeycomb is coated with a composite containing Cr₂ O₃,CeO₂ and Al₂ O₃ and impregnated with Pd using the ammine hydroxide.

This catalyst is prepared exactly as the catalyst of Example IV exceptthat instead of Pt, the slip-coated honeycomb is dipped in a palladiumtetrammine hydroxide solution, yielding a final honeycomb containing,nominally, 0.5 weight percent Pd.

EXAMPLE VI

An alpha-alumina honeycomb is coated with a composition prepared byball-milling a ceria-chromia-alumina powder with a palladium nitratesolution.

A ceria-chromia-alumina powder was prepared and calcined as in ExampleIV, except that instead of milling the powder and depositing it on ahoneycomb for subsequent platinum group metal deposition, a differentprocedure is used. The powder is ball-milled with a solution of Pd(NO₃)₂in distilled water for 17 hours at 114 RPM. It is then diluted with anequal volume of 1 percent (conc.) HNO₃ in water, and this diluted slipis used to dip an alpha-alumina honeycomb having 17 corrugations/inch.After blowing off the excess slurry, the honeycomb is dried at 110°C.,then is calcined 2 hours at 500°C. 7.4 weight percent slip is taken up.The coated block is again dipped in a freshly-prepared slurry ofcomposite, prepared as above. After drying and calcining, weighing showsthe block contains 12.0 weight percent slip, and 0.21 weight percent Pd.

EXAMPLE VII

A ceria-chromia-zirconia-alumina composite is prepared by dissolving22.95 g. of cerium nitrate, 18.56 g. of zirconyl nitrate, and 47.92 g.of chromium nitrate in 587.5 ml. H₂ O for a final volume of 632.5 ml.,and 275 g. of activated alumina powder is added to the solution withconstant agitation for 10 minutes. The slurry is then evaporated todryness with heat and agitation, transferred to a drying oven at 120°C.,and then dried overnight. The dried solids are ground to less than 20mesh and calcined at 970°C. for 1 hour. 5 grams of the composite havinga density of 0.958 g/cc and containing 4% ceria, 4% chromia, 4%zirconia, and 88% alumina is then calcined for 2 hours at 1200°C. It isfound that the surface area after such calcination is 29.9 g/m².

EXAMPLE VIII

191 g. of a ceria-chromia-zirconia-alumina composite as prepared inEXAMPLE VII is transferred to a 1 qt. ball mill jar containing 665 g. ofstones. 191 cc H₂ O plus 14.4 conc. HNO₃ is then added. The whole isthen ball-milled for 19 hours at 66 RPM. The slurry is poured out,diluted with 40 ml. of water to a viscosity of 15 cps, and then used tocoat the same type of honeycomb as in EXAMPLE IV and by the sameprocedure. The catalyst so prepared, after calcination at 500°C.,contains 13 weight percent composite on the total weight of coatedhoneycomb.

EXAMPLE IX

A composite is prepared containing a commercial rare earth mixture,chromia, and alumina. 14.87 grams of a mixture of rare earth nitrates isused. The composition converted to the theoretical oxide content is asfollows: CeO₂ 48%; La₂ O₃ 24%; Nd₂ O₃ 17%; Pr₆ O₁₁ 5%; Sm₂ O₃ 3%; Gd₂ O₃2%; Y₂ O₃ 0.2%; others 0.8%. The rare earth mixture and 3.95 grams ofCrO₃ are dissolved in water and diluted to 80.3 ml. 51 grams of aluminahaving a surface area of 300 m² /g after grinding is added to thesolution with agitation for 5 minutes. The slurry is transferred to anevaporating dish, dried with agitation for one hour under an infraredlamp, transferred to an oven and dried at 110°C. overnight. The driedmixture weighed 65.1 grams containing 10 percent by weight rare earthoxide mixture, 5 percent by weight chromia, and 85 percent by weightalumina. The mixture is crushed to a powder and a 5 gram portion iscalcined at 1200°C. for four hours. The surface area of the calcinedpowder is 43.7 m² /g.

EXAMPLE X

A 1 × 3 inch zircon mullite honeycomb having 12 corrugations per inch iscoated with a composite prepared as in EXAMPLE IX except a two kilogrambatch is prepared and chromium nitrate is used in place of CrO₃. Afterthe dried powder is pulverized, it is calcined at 1000°C. for 4 hours toform a composite. 240 grams of the composite is added to a 1 1/4 gallonball mill with about 10 pounds of stones. 432 ml. of water and 18 ml.concentrated nitric acid are added; the slurry is milled for 17 hoursand cooled to 25°C. The slurry has a density of 1.49 grams per cubiccentimeter and a viscosity of 12 cps. 1 percent nitric acid is added toa density of 1.38 grams per cubic centimeter. The slurry is then placedin a container and stirred continuously. The honeycomb is immersed inthe slurry, blown dry and dried at 110°C. overnight. The coatedhoneycomb is calcined for 2 hours at 500°C. and weighed. The honeycombpicks up 15.3 percent composite on the total weight of coated honeycomb.The honeycomb is then immersed in a solution composed of 18 grams of Na₂PdCl₄ dissolved in 51 ml. of water for 1/2 hour. The honeycomb is thenremoved, the excess palladium solution blown off, and the deposit ishydrolyzed in hot sodium bicarbonate solution. The honeycomb thustreated is then washed chloride free and dried at 110°C. overnight. Thefinal weight gain of the honeycomb is 1.05 percent PdO.

EXAMPLE XI

A composite is prepared by coprecipitation. The composition is the sameas that in EXAMPLE X, i.e., 10 percent of a rare earth oxide mixture, 5percent chromia, and 85 percent alumina. 187.7 grams of aluminumnitrate, 7.4 grams of the same rare earth nitrate mixture used inEXAMPLE IX, and 7.9 grams of chromium nitrate are dissolved in series inone liter of water and the solution transferred to a dropping funnel. Asecond solution was prepared containing 400 ml. of ammonium hydroxide(28.3% NH₃) and 1600 ml. water and transferred to a dropping funnel.2000 ml. of water is added to a 6 liter beaker with vigorous mechanicalstirring. The nitrate solution is then added at room temperature to thewater in the beaker over a period of 30 minutes. The ammonia solution isadded concurrently with the nitrate solution at such a rate as to keepthe pH of the slurry in the beaker at 9.0. After complete addition ofthe nitrate solution, it is found that 580 ml. of the ammonia solutionis added. Stirring is continued for 15 minutes after the coprecipitationis complete. The slurry is allowed to stand overnight and then filteredand re-slurried in 2 liters of water. The second slurry is filtered,excess water removed, and dried for four days at room temperature. Thefilter cake is hand ground to a powder, dried for 1 day at roomtemperature, and overnight at 110°C. 42 g. of composite are recovered.The surface area is good after calcination at 1200°C. for 4 hours.

Representative compositions prepared by the same methods as set forth inthe Examples and results obtained after calcination at 1200°C. arereported in TABLE I.

                                      TABLE I                                     __________________________________________________________________________    Composite Surface Areas                                                                % Chemical Composition   Final Calcination                                                             at 1200°C.                           No.                                                                              % Al.sub.2 O.sub.3                                                                  Rare Earth                                                                              IVB    VIB     Time hrs.                                                                           Surface Area                                                                  m.sup.2 /g                            __________________________________________________________________________    1  100    --        --     --     4      8                                    2  93.4  3.3% CeO.sub.2                                                                          3.3% ZrO.sub.2 2     37                                    3  88.4  5.0% CeO.sub.2                                                                          6.6% ZrO.sub.2 2     26.6                                  4  88.4  6.6% CeO.sub.2                                                                          5.0% ZrO.sub.2 2     36                                    5  85.4  6.6% CeO.sub.2                                                                          8.0% ZrO.sub.2 2     40                                    6  80.1  6.6% CeO.sub.2   13.3% Cr.sub.2 O.sub.3                                                                2     24                                    7  85    10% rare earth*  5% Cr.sub.2 O.sub.3                                                                   4     43.7                                  8  85    5.4% CeO.sub.2   5% Cr.sub.2 O.sub.3                                                                   4     35.3                                           2.7% La.sub.2 O.sub.3                                                         1.9% Nd.sub.2 O.sub.3                                                                   2  - 9 85      10% Nd.sub.2 O.sub.3                                                                5% Cr.sub.2 O.sub.3 4 28.9            10 88     4% CeO.sub.2                                                                           4% ZrO.sub.2                                                                         4% Cr.sub.2 O.sub.3                                                                   2     29.9                                  __________________________________________________________________________    CeO.sub.2                                                                          48% Sm.sub.2 O.sub.3                                                                   3.0%                                                            La.sub.2 O.sub.3                                                                   24% Gd.sub.2 O.sub.3                                                                   2.0%                                                            Nd.sub.2 O.sub.3                                                                   17% Y.sub.2 O.sub.3                                                                    0.2%                                                            Pr.sub.6 O11                                                                        5% Others                                                                              .8%                                                        

In the practice of this invention the catalytic compositions areparticularly useful when employed with the high temperature oxidation ofcarbonaceous fuels. For example, they may be used advantageously in amethod employing a catalytically-supported thermal combustion ofcarbonaceous fuel, as more fully described in co-pending applicationSer. No. 358,411, filed May 8, 1973, of W. C. Pfefferle, assigned to theassignee hereof and which application is incorporated by referenceherein. This method includes the essentially adiabatic combustion of atleast a portion of a carbonaceous fuel admixed with air in the presenceof a catalytic composition of this invention at an operating temperaturesubstantially above the instantaneous auto-ignition temperature of thefuel-air admixture but below a temperature that would result in anysubstantial formation of oxides of nitrogen.

Flammable mixtures of most fuels with air are normally such as to burnat relatively high temperatures, i.e., about 3300°F. and above, whichinherently results in the formation of substantial amounts of nitrogenoxides or NO_(x). However, little or no NO_(x) is formed in a systemwhich burns the fuel catalytically at relatively low temperatures.

For a true catalytic oxidation reaction, one can plot temperatureagainst rate of reaction. For any given catalyst and set of reactionconditions, as the temperature is initially increased, the reaction rateis also increased. This rate of increase is exponential withtemperature. As the temperature is raised further, the reaction ratethen passes through a transition zone where the limiting parametersdetermining reaction rate shift from catalytic to mass transfer. Whenthe catalytic rate increases to such an extent that the reactants cannotbe transferred to the catalytic surface fast enough to keep up with thecatalytic reaction rate, the reaction shifts to mass transfer control,and the observed reaction rate changes much less with furthertemperature increases. The reaction is then said to be mass transferlimited. In mass transfer controlled catalytic reactions, one cannotdistinguish between a more active catalyst and a less active catalystbecause the intrinsic catalyst activity is not determinative of the rateof reaction. Regardless of any increase in catalytic activity above thatrequired for mass transfer control, a greater catalytic conversion ratecannot be achieved for the same set of conditions.

It has been discovered that it is possible to achieve essentiallyadiabatic combustion in the presence of a catalyst at a reaction ratemany times greater than the mass transfer limited rate. That is,catalytically-supported, thermal combustion surmounts the mass transferlimitation. If the operating temperature of the catalyst is increasedsubstantially into the mass transfer limited region, the reaction rateagain begins to increase exponentially with temperature. This is anapparent contradiction of catalytic technology and the laws of masstransfer kinetics. The phenomena may be explained by the fact that thecatalyst surface and the gas layer near the catalyst surface are above atemperature at which thermal combustion occurs at a rate higher than thecatalytic rate, and the temperature of the catalyst surface is above theinstantaneous auto-ignition temperature of the fuel-air admixture(defined hereinbelow). The fuel molecules entering this layerspontaneously burn without transport to the catalyst surface. Ascombustion progresses, it is believed that the layer becomes deeper. Thetotal gas is ultimately raised to a temperature at which thermalreactions occur in the entire gas stream rather than only near thesurface of the catalyst. At this point, the thermal reactions continueeven without further contact of the gas with the catalyst as the gaspasses through the combustion zone.

The term "instantaneous auto-ignition temperature" for a fuel-airadmixture as used herein and in the appended claims is defined to meanthat the ignition lag of the fuel-air mixture entering the catalyst isnegligible relative to the residence time in the combustion zone of themixture undergoing combustion.

This method can employ an amount of fuel equivalent in heating value ofabout 300-1000 pounds of propane per hour per cubic foot of catalyst.There is no necessity of maintaining fuel-to-air ratios in the flammablerange, and consequently loss of combustion (flame-out) due to variationsin the fuel-to-air ratio is not as serious a problem as it is inconventional combustors.

The adiabatic flame temperature of fuel-air admixtures at any set ofconditions (e.g., initial temperature and, to a lesser extent, pressure)is established by the ratio of fuel to air. The admixtures utilized aregenerally within the inflammable range or are fuel-lean outside of theinflammable range, but there may be instances of a fuel-air admixturehaving no clearly defined inflammable range but nevertheless having atheoretical adiabatic flame temperature within the operating conditionsof the invention. The proportions of the fuel and air charged to thecombustion zone are typically such that there is a stoichiometric excessof oxygen based on complete conversion of the fuel to carbon dioxide andwater. Preferably, the free oxygen content is at least about 1.5 timesthe stoichiometric amount needed for complete combustion of the fuel.Although the method is described with particularity to air as thenon-fuel component, it is well understood that oxygen is the requiredelement to support proper combustion. Where desired, the oxygen contentof the non-fuel component can be varied and the term "air" as usedherein refers to the non-fuel components of the admixtures. The fuel-airadmixture fed to the combustion zone may have as low as 10 percent freeoxygen by volume or less, which may occur, for example, upon utilizationas a source of oxygen of a waste stream wherein a portion of this oxygenhas been reacted. In turbine operations, the weight ratio of air to fuelcharged to the combustion system is often above about 30:1 and someturbines are designed for air-to-fuel ratios of up to about 200 ormore:1.

The carbonaceous fuels may be gaseous or liquid at normal temperatureand pressure. Suitable hydrocarbon fuels may include, for example, lowmolecular weight aliphatic hydrocarbons such as methane, ethane,propane, butane, pentane; gasoline; aromatic hydrocarbons such asbenzene, toluene, ethylbenzene, xylene; naphtha; diesel fuel; jet fuel;other middle distillate fuels; hydrotreated heavier fuels; and the like.Among the other useful carbonaceous fuels are alcohols such as methanol,ethanol, isopropanol; ethers such as diethylether and aromatic etherssuch as ethylphenyl ether; and carbon monoxide. In burning diluted fuelscontaining inerts, for example, low BTU coal gas, fuel-air admixtureswith adiabatic flame temperatures within the range specified herein maybe either fuel rich or fuel lean. Where fuel rich mixtures are utilized,additional air or fuel-air admixture may be added to the catalyst zoneeffluent to provide an overall excess of air for complete combustion offuel components to carbon dioxide and water. As stated previously,thermal reactions continue beyond the catalyst zone, provided theeffluent temperature is substantially above the instantaneousauto-ignition temperature.

The fuel-air admixture is generally passed to the catalyst in thecombustion zone at a gas velocity prior to or at the inlet to thecatalyst in excess of the maximum flame propagating velocity. This maybe accomplished by increasing the air flow or by proper design of theinlet to a combustion chamber, e.g., restricting the size of theorifice. This avoids flashback that causes the formation of NO_(x).Preferably, this velocity is maintained adjacent to the catalyst inlet.Suitable linear gas velocities are usually above about three feet persecond, but it should be understood that considerably higher velocitiesmay be required depending upon such factors as temperature, pressure,and composition. At least a significant portion of the combustion occursin the catalytic zone and may be essentially flameless.

The carbonaceous fuel, which when burned with a stoichiometric amount ofair (atmospheric composition) at the combustion inlet temperatureusually has an adiabatic flame temperature of at least about 3300°F., iscombusted essentially adiabatically in the catalyst zone. Although theinstantaneous auto-ignition temperature of a typical fuel may be belowabout 2000°F., stable, adiabatic combustion of the fuel below about3300°F. is extremely difficult to achieve in practical primarycombustion systems. It is for this reason that even with gas turbineslimited to operating temperatures of 2000°F., the primary combustion istypically at temperatures in excess of 4000°F. As stated above,combustion in this method is characterized by using a fuel-airadmixture, having an adiabatic flame temperature substantially above theinstantaneous auto-ignition temperature of the admixture but below atemperature that would result in any substantial formation of NO_(x).The limits of this adiabatic flame temperature are governed largely byresidence time and pressure. Generally, adiabatic flame temperatures ofthe admixtures are in the range of about 1700°F. to 3200°F., andpreferably are about 2000°F. to 3000°F. Operating at a temperature muchin excess of 3200°F. results in the significant formation of NO_(x) evenat short contact times; this derogates from the advantages of thisinvention vis-a-vis a conventional thermal system. A higher temperaturewithin the defined range is desirable, however, because the system willrequire less catalyst and thermal reactions are an order of magnitude ormore faster, but the adiabatic flame temperature employed can depend onsuch factors as the desired composition of the effluent and the overalldesign of the system. It thus will be observed that a fuel which wouldordinarily burn at such a high temperature as to form NO_(x) issuccessfully combusted within the defined temperature range withoutsignificant formation of NO_(x).

The catalyst used in this method generally operates at a temperatureapproximating the theoretical adiabatic flame temperature of thefuel-air admixture charged to the combustion zone. The entire catalystmay not be at these temperatures, but preferably a major portion oressentially all, of the catalyst surface is at such operatingtemperatures. These temperatures are usually in the range of about1700°-3200°F., preferably about 2000°F. to about 3000°F. The temperatureof the catalyst zone is controlled by controlling the combustion of thefuel-air admixture, i.e., adiabatic flame temperature, as well as theuniformity of the mixture. Relatively higher energy fuels can be admixedwith larger amounts of air in order to maintain the desired temperaturein a combustion zone. At the higher end of the temperature range,shorter residence times of the gas in the combustion zone appear to bedesirable in order to lessen the chance of forming NO_(x).

The residence time is governed largely by temperature, pressure, andspace throughput; and generally is measured in milliseconds. Theresidence time of the gases in the catalytic combustion zone and anysubsequent thermal combustion zone may be below about 0.1 second,preferably below about 0.05 second. The gas space velocity may often be,for example, in the range of about 0.5 to 10 or more million cubic feetof total gas (standard temperature and pressure) per cubic foot of totalcombustion zone per hour. For a stationary turbine burning diesel fuel,typical residence times could be about 30 milliseconds or less; whereasin an automotive turbine engine burning gasoline, the typical residencetime may be about 5 milliseconds or less. The total residence time inthe combustion system should be sufficient to provide essentiallycomplete combustion of the fuel, but not so long as to result in theformation of NO_(x).

A method employing the catalyst of the present invention is exemplifiedin a series of runs in which the fuel is essentially completelycombusted, and a low emissions effluent produced. The combustion systemcomprises a source of preheated air supplied under pressure. A portionof the air is passed through a pipe to the combustion zone, and theremainder is used to cool and dilute the combustion effluent. Unleadedgasoline fuel is atomized into the air passing to the combustion zonecountercurrent to the air flow to insure intimate mixing.

In the first series of runs, the catalyst is of the monolithic,honeycomb-type having a nominal 6-inch diameter and is disposed within ametal housing as two separate pieces having parallel flow channels 21/4inches to length extending therethrough. There is a small space of about1/4 inch between these pieces. Both pieces of catalyst haveapproximately 100 flow channels per square inch of cross-section withthe walls of the channels having a thickness of 10 mils. The catalystshave similar compositions and are composed of a zircon mullite honeycombsupport which carries a composite coating of alumina, chromia, and ceriacontaining palladium.

The catalyst for these runs is made by slurrying 2400 grams of activatedalumina powder, less than 40 mesh in size, in a mixer with a solutionprepared by dissolving 2526 grams Cr(NO₃)₃.9H₂ O and 1382 gramsCe(NO₃)₃.6H₂ O in 890 ml. H₂ O. The mixture is dried at 120°C. over aweekend. The dried solids are crushed and screened to less than 40 mesh,and then the powder is calcined for four hours at 1000°C. to form thecomposite of this invention. 3200 grams of the composite is charged to a3.4 gallon ball mill along with 3200 ml. H₂ O and 145.4 grams ofpalladium nitrate. The mill is rolled for 17 hours at 54 RPM. Theresulting slurry has a density of 1.63 grams per ml., a pH of 4.20 and aviscosity of 12 centiposes. 1625 grams of the as-recovered slurry arediluted with 1180 ml. of a 1 percent nitric acid solution. The zirconmullite honeycomb is dipped in the diluted slurry and held for oneminute, and then withdrawn from the slip and blown with air to removethe excess. The coated honeycomb is dried for 16 hours at 110°C. andthen calcined for 2 hours at 500°C. The honeycomb is cooled, and showeda pickup of 11.0 weight percent composition.

The upstream or initial catalyst in the housing has a catalytic coatingwhich comprises 13.9 weight percent of the catalyst. This coating is 70weight percent alumina, 14 weight percent Cr₂ O₃ and 16 weight percentCeO₂ based on these components. The catalyst also contains 0.23 weightpercent palladium (calculated) disposed in the composite. Thesubsequent-in-line catalyst has a similar coating of alumina, ceria, andchromia which is 11.0 weight percent of the catalyst. The catalyst alsocontains 0.18 weight percent palladium (calculated) disposed in thecoating.

Provision is made for contacting the fuel mixed with a portion of thetotal air stream with the catalyst. That portion of the total air streamnot mixed with the fuel is added to the combustion effluent immediatelyupon its exit from the catalyst zone. This dilution or secondary aircools the combustion effluent and samples of the mixture are taken foranalysis. Thermocouples are located adjacent the initial catalyst inletand at the sampling position to detect the temperatures of theselocations.

The catalysts are brought to reaction temperature by contact withpreheated air, and subsequent contact with the air-fuel mixture whichcauses combustion and raised the catalyst temperature further. Theresults obtained using this system during two periods of operation inaccordance with the present invention are reported in TABLE II below asRuns A and B, respectively.

The same reaction system and procedures are used in additionalcombustion runs employing different catalyst pieces that are disposed inthe combustion zone to provide a thermal reaction space between thepieces. The catalysts have zircon mullite honeycomb supports and theinitial catalyst has about 600 parallel gas flow channels per squareinch of cross-section, while the second catalyst has about 100 channelsper square inch. The gas flow path length of the first catalyst is twoinches and of the second catalyst is one inch. The free space betweenthe catalysts is 15/8 inches in the direction of gas flow.

The catalysts are nominally 6 inches in diameter and are made asdescribed above for the catalysts used in Runs A and B. Both catalystscontain a composite coating comprising 70 weight percent alumina, 16weight percent CeO₂ and 14 weight percent Cr₂ O₃, based on thesecomponents. The composite coating for the initial catalyst comprises13.5 weight percent along with 0.26 weight percent palladium dispersedin the composite, and the composite coating for the second catalyst is15.5 weight percent having 0.25 weight percent palladium dispersed init. The results obtained using this system during two periods ofoperation in accordance with the present invention are reported in TABLEII, below as Runs C and D, respectively.

The data of TABLE II illustrate the effectiveness of the process of thisinvention in providing essentially complete combustion of relativelylarge quantities of fuel for a given amount of catalyst. No flashback isencountered in these runs, and the combustion effluents are exceedinglylow in materials that are considered to be undesirable atmosphericpollutants, including nitrogen oxides.

                                      TABLE II                                    __________________________________________________________________________    Combustion Results                                                            RUN                      A      B      C      D                               __________________________________________________________________________    Reactions Conditions                                                          Fuel rate, pounds per hour                                                                             62     124    72     114                             Total air rate, pounds per second                                                                      0.76   1.4    1.2    1.8                             Amount of air mixed with fuel, pounds per second                                                       0.61   1.1    0.7    1.1                             Amount of dilution air, pounds per second                                                              0.15   0.3    0.5    0.7                             Pressure of air stream, atmospheres                                                                    1.9    2.9    3.0    4.3                             Nominal air velocity approaching catalyst inlet,                               feet per second         40     50     35     35                              Fuel-air mixture temperature, °F.                                                               695    785    880    815                             Catalyst temperature, °F.                                               (estimated by radiation pyrometry)                                                                    2310   2470   2430   2400                            Temperature of diluted combustion effluent, °F.                                                 1700-2100                                                                            1800-2200                                                                            1800-2100                                                                            1900-2100                       Analysis of Diluted Combustion Effluent, ppmv                                 NO.sub.x                 0.2    --     0.7    --                              CO                       85     43     13     12                              Hydrocarbons (reported on propane basis)                                                               6      4      3.5    7                               __________________________________________________________________________

The catalysts of this invention can also be used for selected oxidationreactions at lower temperatures. In a typical oxidation they can beemployed to promote the reaction of various chemical feedstocks bycontacting the feedstock or compound with the catalyst in the presenceof free oxygen preferably molecular oxygen. Although some oxidationreactions may occur at relatively low temperatures, many are conductedat elevated temperatures of about 150°C. to 900°C., and generally, thesereactions occur with the feedstock in the vapor phase. The feedsgenerally are materials which are subject to oxidation and containcarbon, and may, therefore, be termed carbonaceous, whether they areorganic or inorganic in character. The catalysts of this invention areparticularly useful in promoting the oxidation of hydrocarbons,oxygen-containing organic components, for example, aldehydes, organicacids, and other intermediate products of combustion, such as carbonmonoxide, and the like. These materials are frequently present inexhaust gases from the combustion of carbonaceous fuels, and thus thecatalysts of the present invention are particularly useful in promotingthe oxidation of such materials thereby purifying the exhaust gases.Such oxidation can be accomplished by contacting the gas stream with thecatalyst and molecular or free oxygen. The oxygen may be present in thegas stream as part of the effluent, or may be added as air or in someother desired form having a greater or lesser oxygen concentration. Theproducts from such oxidation contain a greater weight ratio of oxygen tocarbon than in the material subjected to oxidation and in the case ofexhaust purification these final oxidation products are much lessharmful than the partially oxidized materials. Many such reactionsystems are known in the art.

What is claimed is:
 1. A catalyst composition characterized by a surfacearea of at least 20 m² /g after calcination for two hours at 1200°C.consisting essentially of (a) a catalytically-active, calcined compositeof alumina, a rare earth metal oxide and a metal oxide selected from thegroup consisting of an oxide of chromium, tungsten, a Group IVB metal,and mixtures thereof, and (b) a catalytically-effective amount of aplatinum group metal added thereto after calcination of said compositeat a temperature of at least 850°C.
 2. A composition as defined in claim1 wherein said composite contains about 50 to 96 weight percent of saidalumina, about 2 to 25 weight percent of said rare earth oxide and about2 to 25 weight percent of said metal oxide.
 3. A composition as definedin claim 2 wherein said rare earth oxide is ceria and wherein said metaloxide is chromia.
 4. A composition as defined in claim 1 wherein saidplatinum group metal is present in an amount of about 0.1 to 20 percentby weight of said composite and is selected from the group consisting ofplatinum, palladium, platinum-palladium alloys, and mixtures thereof. 5.A composition as defined in claim 1 wherein said composite ispellet-form.
 6. A catalyst composition consisting essentially of (a) aninert support (b) a catalytically-active calcined composite deposited onsaid support, said composite consisting essentially of alumina, a rareearth metal oxide, and a metal oxide selected from the group consistingof an oxide of chromium, tungsten, a Group IVB metal, and mixturesthereof, said composite having been calcined at a temperature of atleast 850°C. before deposition on said support and characterized by asurface area of at least 20 m² /g after calcination for two hours at1200°C., and (c) a catalytically-effective amount of a platinum groupmetal incorporated in said composite after deposition of said compositeon said support.
 7. A catalyst composition as defined in claim 6 whereinsaid support is pellet-form.
 8. A catalyst composition as defined inclaim 6 wherein said support is a ceramic honeycomb.
 9. A catalystcomposition as defined in claim 6 wherein said platinum group metal ispresent in an amount of about 0.2 to 10 percent by weight of saidcomposite and wherein said platinum group metal is selected from thegroup consisting of platinum, palladium, platinum-palladium alloys, andmixtures thereof.
 10. A catalyst composition as defined in claim 6wherein said composite contains about 50 to 96 weight percent of saidalumina, about 2 to 25 weight percent of said rare earth oxide, andabout 2 to 25 weight percent of said metal oxide.
 11. A catalystcomposition as defined in claim 6 wherein said rare earth oxide is ceriaand wherein said metal oxide is chromia.
 12. A method for thepreparation of a catalyst composition consisting essentially of (a)forming an intimate admixture consisting essentially of analumina-producing aluminum compound, a rare earth compound which uponcalcining yields the corresponding oxide and at least one metal compoundwhich upon calcining yields the corresponding oxide wherein said metalcompound is selected from the group consisting of a compound ofchromium, tungsten, and a Group IVB metal and mixtures thereof, (b)calcining said intimate admixture at a temperature of at least 850°C. toform a catalytically-active composite containing alumina and theaforesaid rare earth and metal oxides, and characterized by a surfacearea of at least 20 m² /g after calcination for two hours at 1200°C.,and (c) incorporating a catalytically-effective amount of a platinumgroup metal to said calcined admixture.
 13. A method as defined in claim12 wherein said aluminum compound is alumina.
 14. A method as defined inclaim 12 further comprising depositing said composite on a relativelyinert substrate to form a coating thereon prior to step (c).
 15. Amethod as defined in claim 14 further comprising forming said intimateadmixture into pellets before calcining.
 16. A method as defined inclaim 12 further comprising depositing said catalyst composition on arelatively inert substrate to form a coating after step (c).
 17. Amethod as defined in claim 12 wherein said metal compound is chromium.18. A method as defined in claim 12 wherein said intimate admixture isformed by coprecipitation of said compounds from an essentially aqueoussolution thereof.
 19. A method as defined in claim 18 wherein saidcoprecipitation is accomplished by adding an effective amount of watersoluble base to said aqueous solution.
 20. A method as defined in claim18 further comprising depositing said composite on relatively inertsubstrate to form a coating thereon prior to step (c).